One of the major costs of solar water heating systems is the relatively large volume heated liquid storage tank which stores liquid heated by solar energy after circulation through a solar panel. While in general the stored liquid may be water, it is also frequently an alcohol, or ethylene glycol, and water mixture, or even a light oil. If the liquid is water it may be directly connected to a domestic water supply. Usually, however, the heated liquid is arranged to heat domestic water indirectly as by a heat exchange coil immersed in the stored liquid (so that potability of the liquid is not required).
Tanks for solar liquid storage have been constructed of copper, stainless steel, galvanized iron and plastic. The prime requirement for such a tank is a useful life of 10 to 20 years without maintenance. It must also be relatively lightweight for a passive solar water heating system. Such a system requires close proximity between the hot water storage tank and the solar absorber panel. The two units may be constructed either in a tank-over-panel configuration, or in a back-to-back arrangement as disclosed in my U.S. Pat. Nos. 4,137,964 issued Feb. 6, 1979, 4,245,617 issued Jan. 20, 1981 and 4,258,701 issued Mar. 31, 1981. In the latter system a one-way valve between the solar panel and the storage tank prevents back siphoning of heated liquid from the tank when liquid in the solar panel is cooler. In either case, the storage tank is placed on a roof, or other not easily accessible location, where the solar panel will receive maximum solar energy. Accordingly the weight loading of the unit on the roof is frequently critical. For this reason, the tank is desirably strong, lightweight and durable for a given liquid volume.
While in general a group of parallel pipe members, assembled with suitable headers, can be formed of plastic to form such a tank, having adequate flexural strength, the total liquid volume of the tank is low as compared to a tank with a generally rectangular cross-sectional area. On the other hand, such a rectangular tank is generally deficient in strength unless the walls are of adequate thickness or reinforced to withstand stresses applied by the hydrostatic pressure within the tank. Accordingly, there has been a need for a lightweight, inexpensive tank having adequate strength to volume ratio.
Tanks have also been constructed of two sheets of plastic in which multiple discrete areas between the sheets are secured together, but such tanks must be limited in volume to surface area unless the tank is externally supported.
Accordingly there is a pressing need for a tank having a large volume to surface area ratio which will be resistant to flexure.